Polyesters based on terephthalates, particularly copolyesters including ethylene glycol and 1,4-cyclohexanedimethanol (CHDM), are important commercial polymers used in the production of plastic articles such as packaging materials, molded articles, and films. The manufacturing process historically utilized in the synthesis of these copolyesters involves an initial ester exchange reaction wherein dimethyl terephthalate, ethylene glycol, and CHDM are reacted in the presence of a suitable catalyst with removal of by-product methanol. The product of this reaction is polycondensed under reduced pressure and high temperatures to produce the final product.
One difficulty encountered in the manufacture of these copolyesters is that the final product has a slightly yellowish tint. A neutral hue with glass-like appearance is highly desirable. For applications where these polymers are formed into thick sheet articles, the yellowish tint is particularly objectionable. In addition to a neutral hue, for certain applications a relatively high inherent viscosity (IV) is desired, e.g., for heavy gauge sheeting. It is also desirable economically to produce these copolyesters using terephthalic acid instead of dimethyl terephthalate.
Attempts to prepare such copolyesters with terephthalic acid following prior art teachings regarding conditions for the esterification reaction have, until recently, generally resulted in an esterification product having a diminished activity in the subsequent polycondensation step. The diminished activity is observed when comparing the polycondensation reaction starting with an ester exchange product prepared using dimethyl terephthalate as a reactant instead of an esterification product prepared using terephthalic acid. Diminished activity results in a lower viscosity.
For instance, U.S. Pat. No. 4,020,049 discloses a process for preparing linear polyesters from a dicarboxylic acid and glycols. Feed mole ratios of glycol to dicarboxylic acid of 1.05:1 to 1.7:1 are specified for the esterification reaction. Terephthalic acid and ethylene glycol are added continuously and simultaneously in stoichiometric proportions to produce a low polymer, which is removed to a polymerization vessel for polycondensation. In this reference, it is said that the resin produced has an intrinsic viscosity of at least 0.40, generally above 0.50. The highest reported intrinsic viscosity is 0.62.
U.S. Pat. No. 5,198,530 discloses a process for preparing polyesters by esterification of terephthalic acid with CHDM. This reference is only concerned with copolyesters having at least 80 mole percent 1,4-cyclohexanedimethylene terephthalate units. The process uses feed mole ratios of glycol to dicarboxylic acid of 1.0:1 to 1.5:1 in the esterification reaction, and requires a catalyst in the esterification reactor. At least three reactors connected in series may be used, each corresponding to, respectively, an esterification step, a precondensation step, and a polycondensation step. In addition to the feed of diol, additional CHDM is fed into the vapor space above the esterification product, in the reactor prior to the precondensation step.
U.S. Pat. Nos. 5,340,907 and 5,385,773 to Yau et al. are directed to a copolyester including cyclohexanedimethanol and process for production of the copolyester, using a catalyst and inhibitor system consisting essentially of Mn, Zn, Ti, Ge, and P. There are several examples using dimethyl terephthalate in batch-wise production of the copolyester; there are no examples using terephthalic acid itself. The reference further teaches that a continuous process can be operated by adding free dicarboxylic acid and glycol to molten low molecular weight linear or branched polyester resin and reacting them while continuously withdrawing low molecular weight resin and introducing the resin withdrawn into a polymerization apparatus.
Recently, a process showing improved polycondensation activity using terephthalic acid has been described in U.S. Pat. No. 5,681,918, to Hataway et al. The process comprises the steps of:
(1) reacting terephthalic acid, ethylene glycol, and 1,4-cyclohexanedimethanol in a feed mole ratio of total glycols to dicarboxylic acid of 1.7:1 to 6.0:1, at a temperature of 240.degree. C. to 280.degree. C., and a pressure of 15 psig to 80 psig for 100 to 300 minutes to form an esterification product; PA1 (2) adding a polycondensation catalyst and 0.1 to 40 ppm of a toner to the esterification product of Step (1), wherein the polycondensation catalyst is selected from the group consisting of titanium, germanium, antimony, and combinations thereof; and PA1 (3) polycondensing the product of Step (2) at a temperature of 260.degree. C. to 290.degree. C. and a reduced pressure of 400 mm Hg to 0.1 mm Hg for a sufficient time to form a copolyester having an inherent viscosity of at least 0.50 dL/g, said process comprising adding 10 to 100 ppm of a phosphorus stabilizer in Step (2) or in Step (3). This process is an important advance, since the polycondensation activity of the esterification product is vastly improved over prior art methods using terephthalic acid.
However, under certain circumstances it may be desirable to have a low feed mole ratio of total diols to total dicarboxylic acids into the first esterification reactor. Such circumstances might include a desire for lower energy consumption or lower by-product formation. Therefore, in a process of making copolyesters involving an esterification step and a polycondensation step, there is still the problem of attaining a high viscosity resin providing thick sheets having a neutral hue, without using high feed mole ratios into the first esterification reactor.